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Designing in the case of a complex repair of a structure by means of shotcrete
Designing stage is the key to a successful repair. Here detailed requirements concerning:
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the method of base preparing,
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shotcrete in terms of its physio-mechanical properties (compressive strength, base adhesion, absorbability, water resistance, frost resistance),
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shotcrete in terms of its state after application
should be determined.
When assessing the level of damage we distinguish the following types:
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structure’s load-bearing capacity at risk (critical damage);
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users’ safety at risk (e.g. loosening/spalling of concrete elements from the structure’s surface);
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concrete lost its protective properties against steel.
Classification of damages threatening the structure’s load-bearing capacity depends upon the definition of damage due to usability or load capacity of the structure. It will be different for pre-stressed structures and reinforced concrete. Currently it is commonly assumed that the anticipated period of use for a structure ends at when reinforcement starts to corrode. The period is dependable among others on thickness of concrete surround, its quality and type of environment that influences the structure.
At a certain stage it is too costly to repair local damages with the PCC system as their size covers significant area of the structure and therefore it is more reliable and efficient to spray adequate repair material onto the whole surface.
The idea behind shotcrete technology is to restore structure’s original properties by reconstructing the reinforcement surround, filling cavities, repairing the structure and recover the structure’s geometry.
Table 1 shows types of damages and recommended repair techniques.
Complex repair by means of shotcrete gives the structure an aesthetic look, particularly when you use the following finishing technique:
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‘as shot’ – state as after spraying of concrete,
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re-spraying (after initial setting) of material in order to obtain desired surface quality ‘cut & flash’,
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smooth finishing – trowelling (should be avoided due to micro-cracking).
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Surface damage
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Recommended repair method
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No loosening, honeycombing or spalling is visible on concrete. The concrete is uniform, insignificant carbonisation, insufficient surround, no unevenness after forming. Tearing force higher than 1.5 MPa.
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Anti-corrosion protection of concrete with CO2 diffusion resistant coating (e.g. acrylic dispersion).
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Non-uniform concrete, visible aggregate fractionation, unevenness after forming, insignificant carbonisation, insufficient surround. Tearing force higher than 1.5 MPa.
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PCC stopping, concrete anti-corrosion protection- as above.
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Loosening of concrete proves reinforcement corrosion, concrete carbonated, locally at the depth of reinforcement.
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Local manual PCC plastering, PCC stopping, concrete anti-corrosion protection- as above.
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Carbonisation at the depth of reinforcement. Loosening and cracking of concrete on significant areas. Tearing force higher than 1.5 MPa. In places to be chipped off less than 1.5 MPa.
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After a mechanical or hydrodynamic chipping off, filling of cavities and re-profiling with sprayed concrete of the whole surface (the best would be concrete modified with active SiO2 and migrating corrosion inhibitors MCI).
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Advanced carbonisation, main reinforcement corroded. Tearing force less than 1.5 MPa on dominating area.
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After a mechanical or hydrodynamic chipping off mounting of reinforcement mesh. Repair with sprayed concrete modified with active SiO2 and migrating corrosion inhibitors MCI- thickness min. 5 cm.
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Very advanced carbonisation. Main reinforcement corroded in more than 30% of section’s area. Significant scratches and cracks of structure. Tearing force lower than 1.5 MPa. Pre-failure state. Fig. 4.
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Repair by means of increasing of the cross-section (structural sprayed concrete). Additional structural reinforcement calculated depending on the element’s load capacity. Thickness of sprayed concrete up to 0.5 cm as needed. Repair method - as above.
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Designing of shotcrete mixes
Development of the composition of a mixture must meet design requirements that result from the type of repair to be carried out, the function and purpose of structure, and factors that influence the structure. Depending on the needs, shotcrete may be composed in many ways.
Here are some types:
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traditional, on plain cements and aggregates with addition of flexibilisers, silica dust, set accelerators, fly ashes and others,
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reinforced with steel or polyurethane fibres etc. i.e. not classical shotcrete but fibre concrete,
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fire-resistant which includes heat-resistant ingredients such as fire resistant fillers of adequate grain size or fire resistant new generation mixes of low cement content.
Shotcrete mixes include also additives that improve properties of such concrete. These additives are protected with patents, as they are the latest achievement of construction-related chemistry branch.
Another important additive to spraying mixtures is silica dust that improves technological and usability properties (in both processes) due to acceleration of concrete setting and hardening, as it increases strength and resistance to impact of aggressive chemical factors and improves cohesion, which results in decrease of mortar loss during spraying. The possibility of minimising the loss of material is of particular importance, as we know the main drawback of shotcrete method is losses caused by ricochets off the receiving surface.
Mixes are developed according to application requirements. It is worth mentioning that the latest material solutions allow repairing surfaces with increased contents of chlorides. For many reasons chlorides cannot be entirely eliminated from structures so far. Protection of reinforcement in the base on which concrete was sprayed is possible thanks to the so-called corrosion inhibitors systems.
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